1. Field of the Invention
The present invention relates to a method for estimating a state of charge (SOC) of a secondary battery, such as a nickel-metal hydride (Ni—MH) battery or the like, that is installed in a pure electric vehicle (PEV), a hybrid electric vehicle (HEV) or the like as a power source for a motor and a drive source for various loads.
2. Description of the Related Art
Conventionally, in an HEV, when the output of its engine is higher than the required power for driving, a generator is driven with the surplus power to charge a secondary battery. On the contrary, when the output of the engine is lower than the required power, a motor is driven with the electric power of the secondary battery to output power to cover the shortage of the required power. In this case, the secondary battery is discharged. Therefore, when a secondary battery is mounted into an HEV or the like, it is necessary to control the charge and discharge of the secondary battery to maintain an appropriate driving state of the vehicle.
To achieve this, the state of charge (hereinafter referred to as SOC) of the secondary battery is estimated by calculation based on the detected battery voltage, current, temperature, and the like, and the SOC is controlled so that the fuel consumption efficiency of the vehicle is maximized. In this case, the SOC level generally is controlled to fall within the range of, for example, 50% to 70% in order to optimize the balance between power assist by motor drive during acceleration and energy regeneration during deceleration (regenerative control). Specifically, when the SOC is lowered to, for example, 50%, charge is predominantly performed. Conversely, when the SOC is increased to, for example, 70%, discharge is predominantly performed. As a result, the SOC is controlled to be within the range mentioned above.
In order to accurately perform such SOC control, it is necessary to accurately estimate the SOC of secondary battery that is being charged/discharged. There is the following known conventional method for estimating SOC from battery voltage.
Initially, a plurality of sets of pair data of a voltage V and a charged/discharged current I are obtained and stored over a predetermined period of time. The pair data is used to perform regression analysis to obtain an approximate first-order straight line (voltage V-current I approximate straight line). The V intercept of the V-I approximate straight line is obtained as a battery voltage V0 (no-load voltage). In addition, an integrated value ∫I of the current I is calculated; a polarized voltage Vp of a battery is obtained using a function of a temperature T, the battery voltage V0 and the current integrated value ∫I; and an electromotive force E of the battery is obtained by subtracting polarized voltage Vp from battery voltage V0. Next, an SOC is estimated from the obtained electromotive force E with reference to previously prepared electromotive force-SOC characteristics.
When an SOC is estimated from voltage in the above-described manner and a possible input/output voltage during a predetermined period of time is calculated, it is necessary to estimate the polarized voltage accurately, or eliminate the influence of polarized voltage. A battery voltage is increased during charge and is decreased during discharge, and the difference therebetween is referred to as a polarized voltage.
The influence of polarized voltage is eliminated, for example, as follows. After the charge and discharge of a battery is stopped, the duration of the stopped state is measured. When the duration reaches a given time, at which an open-circuit voltage can be considered to be equal to a voltage corresponding to the actual charged state of the battery, immediately before the start of charge and discharge of the battery by switching on an ignition, the open-circuit voltage of the battery is detected based on an output signal output by a voltage sensor. Based on the detected open-circuit voltage, an SOC is determined (see, for example, JP 2002-365347A).
However, the above-described conventional SOC estimation technique has the following problems.
It is difficult to estimate a polarized voltage accurately. Therefore, when an SOC is estimated based on a battery voltage measured during driving a vehicle as in a technique disclosed in JP 2003-197275A, the resultant SOC contains a number of errors, i.e., the SOC cannot be estimated with high accuracy. In the technique of JP 2002-365347A, it takes a long time (e.g., about one month) to wait until the influence of polarized voltage occurring when a battery is charged and discharged is completely eliminated. Therefore, this technique is not sufficiently practicable.